Obesity is more prevalent in persons with intellectual disabilities (ID) than in the general population. Weight loss in this population is particularly important during adolescence, a critical period in which parents can impart healthful dietary and physical activity knowledge and skills in anticipation of their child's transition into adulthood ad independent living. Family-based treatments have been effective in promoting weight loss among typically developing children and adolescents, but to our knowledge no randomized controlled trials (RCTs) have been reported with children or adolescents with ID, and research is sorely needed. This R01 proposal seeks to build upon our recently completed RCT (N=21) with adolescents/young adults with Down syndrome in which we found encouraging results favoring what we refer to as family-based behavioral intervention (FBBI) over a purely educational approach. The specific aims of this R01 proposal are to: 1) conduct a needed larger replication of our pilot program (FBBI) with 48 adolescents with ID aged 15-22 years, participating in the program with a parent, randomized to FBBI or delayed treatment (DT); and 2) develop, implement and evaluate a follow-up maintenance intervention (FBBI+MI) designed to sustain weight loss through longer-term lifestyle maintenance strategies. After completion of the FBBI, participants will be randomized to either a FBBI-MI intervention for 6 months or a no treatment period. The primary outcome of this study is change in adolescent participants' weight status, reported as body mass index (BMI). Secondary outcomes are changes in adolescents': fruit, vegetable and high energy-dense food consumption; time spent in moderate to vigorous physical activity; screen time; and self-efficacy. Data are collected at baseline, 6 months (end of FBBI), 12 months (end of FBBI+MI) and 18 months (no-intervention follow-up). Participants assigned to DT begin FBBI after an initial 6-month delay, during which they serve as controls, and therefore receive an additional baseline measurement. Treatment fidelity is assessed by direct observation or audiotape review for 20% of sessions, covering the interventionists' implementation of nutrition education, individual nutrition counseling and parent behavioral training components. The results of this study will contribute to the research on weight reduction for adolescents and inform larger-scale research and clinical interventions.